Image storage device



Dec. 26, 1961 K. H. BUTLER IMAGE STORAGE DEVICE Filed Oct. 31, 1957 l L/4 I7} 1 /Z INVENTOR: lfE/TH H. BUTLfR ATTOPA/[Y States atent 3,015,036 IMAGE STORAGE DEVICE Keith H. Butler, Marblehead, Mass, assignor, by mesne assignments, to Sylvauia Electric Products Inc., Wilmington, Del., a corporation of Delaware Filed Oct. 31, 1957, Ser. No. 693,623 6 Claims. (Cl. 250-213) This invention relates to devices in which an image is produced by incident radiation and will persist for some time afterv the incident radiation ceases or changes. Such devices have a variety of uses, for example in electrical computers and radar screens.

Image-production screens are known in which an electroluminescent layer is used in series with a photoconductive layer, and a source of voltage across the two. Light from the electroluminescent cell, once the latter is excited, can be fed back to the photoconductor to produce a storage of the image. In previous devices of this type, however, the photoconductive layer had to be in either a foraminous or a mosaic form to result in effective storage without blurring the image.

I have discovered, however, that efiective storage can be obtained when the layers of electroluminescent material and photoconductive material are both continuous layers, without being foraminous, if a foraminous webbing of opaque material is used between the two layers.

Clear image reproduction is obtained with such a device, and the image will persist for a considerable period.

The eiiect appears to be due in part to a capacitive effect in the photoconductor, that is the light fed back through the holes makes the photoconductor conductive in the region of an illuminated hole, and although the conduction may not extend all the way through the photoconductive layer, it extends part way through it, thereby reducing the length of dielectric path between electrodes, and keeping the voltage drop low in the photoconductor. There is also a certain amount of photoconductivity throughout the length of the photoconductive layer, that is, a certain amount of volume resistivity. The capacitive eiiect is most pronounced on excitation by radiation in the wavelength range of about 550 to 570 millimicrons, and the volume resistivity effect on excitation by red or infrared radiation, say in the wavelength range of 750 to 790 rnillimicrons.

For such reasons, I call the layer whose impedance varies with incident radiation, a photo-admittive layer, since its admittance changes with light. In an electrical device, admittance is the reciprocal of impedance.

Accordingly, one object of my invention is to provide a device which when momentarily illuminated at one or more spots will start emitting light from these spots and will continue to emit for a period of time.

A further object is to provide such a device of high sensitivity. To achieve that object, I sinter the photoconductive particles to each other and to the foraminous layer. This changes the photoconductive layer from a putty-like layer which can be scraped away to a cerarniclike layer which can only be removed by grinding. The elimination of any binder in the final photo-admittive layer gives a greater density of photoconductive particles and the firm, sintered joinder of the particles insures good contact between particles. These features result in a gain of to 100 times in sensitivity of the photo-admittive layer.

Such sintering can oly be done if the electroluminescent layer also is free from organic binder. The use of a ceramic embedding dielectric for the electroluminescent particles, and a ceramic opaque material for the feedbacklimiting layer, permits the photoconductive particles to be sintered directly to themselves and to the opaque material, that is the particles can be directly sintered onto the feedback-limiting layer.

The effectiveness of the device will also be increased by the use of transparent conductive discs at the bottom of the holes, that is on the electroluminescent layer. The discs can be about the same size as the holes in the layer of opaque material.

Other objects, features and advantages of the invention will be apparent from the following specification, taken in connection with the accompanying drawings, in which:

FIG. 1 is a plan view of one embodiment of the invention, with the various layers broken away in part, so that they can all be shown; and

FIG. 2 is an elevation of the invention, partly in section.

In the figures, the device 1 has the glass base plate 2 with a transparent electrically-conductive coating 4 thereover. The coating 4 can be, for example, a coating of tin, titanium or silicon chloride, applied in manners well known in the art, for example as in copending United States patent application Serial. No. 365,617, now abandoned, filed July 2, 1953 by Richard M. Rulon.

An electroluminescent layer 6 is applied over the transparent conductive layer 4, and can, for example, be an electroluminescent phosphor embedded in a glass or ceramic dielectric materials, for example, as shown in said Rulon application.

The forarninous layer 8 of opaque insulating material is next applied, and may be, for example, a black glass or ceramic layer applied as in United States patent application Serial No. 649,876, now Patent No. 3,001,078, filed April 1, 1957, by Richard M. Rulon. The layer is applied as a complete, continuous layer, without the holes 10 then being present. A foraminous metal mask, for example of copper, and having holes corresponding to those desired in the foraminous layer 4, is then placed over said layer, and an air stream containing particles of talc or other mildly abrasive material, is directed over the copper plate, through a nozzle, for example, onto that layer 4, until the holes 10 are cut in said layer. The action is similar to sand-blasting and gives clear, sharplydefined holse in layer 4. The holes, of course, need not necessarily be round but could for example, be square.

The material of foraminous layer 4 need be opaque only to radiations which excite the photoconductor.

Photoconductive material, for example, copper-sensitized cadmium sulfide, as shown in an application Serial No. 682,122, now Patent No. 2,878,394, filed September 5, 1957, by Frederic Koury, is then applied in a series of several sintered layers, for example four layers, as shown in said Koury application. The sintering can be done at about 550 C. for about 20 minutes, for example. An electrically-conducting grid 14 is then applied, for example, by evaporating a metal such as silver, aluminum, platinum, gold or tin, onto the photoconductive layer 12 in a vacuum in a manner well-known in the art. The evaporation is done in two stages, once through a mask slotted to produce lines 15 and then through a slotted mask with lines at to the first mask, to produce lines 16, thus forming a grid.

An actual wire grid can also be used to form the electrode 14, or a glass plate with a transparent conductive coating in contact with photoconductive layer 12. In the latter case, the conductive coating, being transparent, can be continuous.

If the grid 14 be used, however, the lines 15 and 16 should pass along the space outside of the holes 10, so that they will not interfere with the image.

In other words, the holes 10 should be in register with the openings 17 in electrode 14 for best results.

In operation, a source of voltage, preferably alternating, though pulsed DC. is also satisfactory, is connected between electrodes 14 and 4, the dark impedance of photoconductive layer 12 being high enough for the voltage applied, so that the voltage across electroluminescent layer 6 is too low for appreciable electroluminescence. An image can then be focussed on layer 12 so that its conductivity will vary from point to point with the intensity of the incident light.

if the foraminous layer 8 were not present, the feedback from electroluminescent layer 6 would be complete and would tend to spread the image out and blur it. On the other hand, if the layer 8 were continuous, and did not have the holes 10, there would be no feedback at all and no image storage. The image would fade out when the incident light causing it Was removed.

With the use of holes 10, however, the feedback is confined to particular spots, which keeps the image sharp yet stores it. Transparent conductive discs of about the same size as the holes 10 can be formed on the electroluminescent layer 6 at the bottoms of said holes it). These discs greatly increase the effectiveness of the device. They can be applied in the usual manner, for example by spraying through a mask having appropriate holes. The material used can be the same as that of the conductive coating 4 applied to glass piece 2.

in one specific embodiment of my device a storage time of 65 seconds has been obtained at 300 cycles per second and a voltage of 600 volts R.M.S. In that embodiment, the photoconductive layer 12 about 10 mils thick, and the opaque foraminous layer 8 about 1 mil thick, wit holes 0.040 inch in diameter and about 0.07 inch between centers, giving about 16 holes per inch. Transparent conductive discs were used on the electroluminescent layer as described above. The glass plate 2- Was about 0.08 inch thick, and about 2 by 2 inches in size. The lines 15, 16 were about 15 mils Wide and about 0.4 mil thick.

What I claim is:

1. An electroluminescent image device comprising a continuous electroluminescent layer and a continuous photo-admittive layer separated by a foraminous layer of opaque material, all three layers being sintered together to form an integral unit, and an electrode on each side of said unit.

2. An electroluminescent image device comprising a continuous electroluminescent layer, a continuous photo- 4 admittive layer of photoconductive particles sintered together, a foraminous layer of opaque ceramic material therebetween, and electrodes on opposite sides of the resultant laminar structure.

3. The device of claim 2 in which the electrodes are transmissive of light.

4. The device of claim 3 in which one of the electrodes is on the electroluminescent layer and is transmissive of radiation emitted by said layer, and in which the other electrode is on the photo-admittive layer and is transmissive of radiation affecting the admittance of the photoadmittive layer.

An electroluminescent device comprising an extended electrode, a continuous electroluminescent layer over said electrode, said layer comprising electroluminescent particles embedded in ceramic, a foraminous layer of substantially opaque ceramic material over said electroluminescent layer, a sintered continuous phcto-admittive layer over said foraminous layer, and an electrode over the photo-admittive layer with a grid structure.

6. An electroluminescent device comprising an extended electrode, a continuous layer of electroluminescent particles embedded in ceramic over said electrode, a foraminous layer of substantially opaque mtaerial over said electroluminescent layer, a sintered continuous photoadmittive layer over said foraminous layer, and an extended electrode capable of transmitting radiation over said photo-admittive layer.

References Cited in the tile of this patent UNITED STATES PATENTS 2,406,139 Fink et al. Aug. 20, 1946 2,594,740 De Forest et a1 Apr. 29, 1952 2,764,693 Jacobs et al Sept. 25, 1956 2,768,310 Kazan et al. Oct. 23, 1956 2,773,992 Ullery Dec. 11, 1956 2,792,447 Kazan May 14, 1957 2,805,360 McNaney Sept. 3, 1957 2,884,507 Czipott Apr. 28, 1959 2,897,399 Garwin et al. July 28, 1959 2,930,999 Van Santen et al. Mar. 29, 1960 FOREIGN PATENTS 157,101 Australia June 16, 1954 OTHER REFERENCES Botwell: Journal I.E.E., August, 1957, pp. 454-459. 

